Effects of an extreme winter (1995-96) flood on channel migration

Geomorphic surveys on reaches of eight streams in the Wenatchee National Fo rest, Washington, were made just prior to and immediately following the lar gest flood on 83 years of record during the winter of 1995-96. One objectiv e of this research was to characterize lateral channel migration of channel s over variety of landscapes as a result of the flood. Average lateral chan nel migration of channel bends resulting from the flood ranged from 0.13 me ters (0.023 channel widths) up to a value of 2.01 meters (0.04 channel widt hs). The relationship between the ratio of lateral channel migration to ban kfull width and the degree of channel entrenchment and channel sinuosity is not significant at the 95% confidence level for the eight streams included in this study. A second objective of this research was to compare rates of current lateral channel migration to historic rates of lateral channel mig ration. Historic lateral channel migration measurements were made on six of the eight stream reaches by studying a sequence of aerial photography. A c omparison of historic lateral channel migration rates to the lateral channe l migration rates measured as a result of the 1995-96 flood indicates that lateral channel migration in the studied streams is incrementally continuou s rather than episodic. That is, the average lateral channel migration meas ured by the pre- and post-flood surveys are not significantly different (at the 95% confidence level) from the long-term average rates of lateral chan nel migration.